The present invention relates to processes of making multifilament yarns having properties sufficient for use in medical devices, multifilament yarns prepared by the processes, and medical devices prepared from such multifilament yarns.
Medical devices prepared from multifilament yarns made by extruding polymers containing relatively high levels of L-lactide, e.g. greater than about 80 mole percent, and relatively low levels of glycolide, e.g. less than about 20 mole percent, can retain a majority of their original strength after a 6-month in vivo test. Such polymers have relatively slow rates of crystallization and it is very difficult, using known technology for manufacturing such yarns, to form multifilament yarns prepared from such polymers that possess tensile properties effective for use in manufacturing medical devices, e.g. sutures and surgical meshes.
Conventional processes for the formation of high strength multifilament yarns normally include pre-heating extruded, spun multifilament yarn with a known heating means, such as a hot roll, heated pin or the like, drawing the multifilament yarn in one or more drawing steps, and then immediately annealing the drawn yarn with one or more hot annealing rolls in order to xe2x80x9csetxe2x80x9d, or lock-in, the stretched, i.e. drawn, molecular chains. Once the drawn yarns are annealed, they usually are relaxed slightly to reduce the residual internal stress and/or shrinkage before being cooled and wound onto a bobbin. Such processes are described in, e.g. U.S. Pat. No. 4,003,974, the contents of which is incorporated herein by reference in its entirety.
When such conventional processes were used to prepare multifilament yarns prepared from a polymer comprising relatively high L-lactide levels, e.g. about 95 mole percent, it was discovered that the tenacity and strength required for use in the manufacture of medical devices, e.g. sutures, could not be achieved. For example, when such multifilament yarn was drawn with a conventional 1-stage drawing frame, the process resulted in fibers with undesirable tensile properties, such as low tenacity. Over-drawing of the fibers also led to the formation of excessive broken filaments and thus was not effective in improving the yarn tenacity. The drawn yarn average tenacity of yarns prepared according to such conventional processes could barely meet the minimum requirement of 5.6 g/d required for use in absorbable surgical suture products. Traditional process control methods used to optimize the extrusion conditions also was unsuccessful in consistently providing properties required for use in such medical devices.
It would be advantageous to provide processes utilizing more than one drawing step for making multifilament yarns, such that the yarns exhibit properties useful for use in medical devices, e.g. sutures.
The present invention is directed to processes for the manufacture of multifilament yarn, comprising the steps of extruding and spinning multifilaments of a polymer comprising at least 85 mole percent L-Lactide and not more than 15 mole percent glycolide to form a spun multifilament yarn, passing the spun multifilament yarn through an orientation roll heated at a temperature of less than 98xc2x0 C. and drawing the spun multifilament yarn to a draw ratio of at least 4:1 to form an oriented multifilament yarn, annealing the oriented multifilament yarn by passing the oriented multifilament yarn over an annealing roll at a minimum temperature that is greater than that of the orientation roll and a maximum temperature that is effective to provide finished multifilament yarn having a tenacity of at least 5.6 grams/denier (g/d); and redrawing the annealed, oriented multifilament yarn by at least 10 percent at a temperature lower than the temperature of the annealing roll. Such processes allow the use of relatively simple equipment to make such yarns, while at the same time providing process stability and good product yield. Multifilament yarns manufactured by the processes of the present invention possess significantly higher strength, greater elongation and toughness, when compared to multifilament yarns prepared by conventional processes. Such multifilament yarns are useful as medical devices, e.g. sutures.